Orogenic gold deposits are the main source of gold for humanity. These deposits formed from the Archaean to Phanerozoic at crustal depths of more than 4 km. The key parameters accounting for their formation remain under discussion. I review these parameters in light of recent scientific advances using a systemic approach, where the relative pertinence of each component of a complex system in time and space is addressed. From the base to the top of the mineralising system, the following components and processes are reviewed: (1) the fluid sources; (2) the appropriate ligands and their sources; (3) the solubility of gold and its potential sources; (4) the function of faults as conduits; and (5) the mechanisms for precipitating gold. This review provides further support for the role of carbon-rich and pyritic sedimentary rocks in the formation of orogenic gold deposits throughout geological time.

The Saint-Honoré carbonatite complex hosts a rare earth element (REE) deposit traditionally interpreted as being produced by late-stage hydrothermal fluids that leached REE from apatite or dolomite found in the early units and concentrated the REE in the late-stage units. New evidence from deeper units suggest that the Fe-carbonatite was mineralized by a combination of both magmatic and hydrothermal crystallization of rare earth minerals. The upper Fe-carbonatite has characteristics typical of hydrothermal mineralization—polycrystalline clusters hosting bastnäsite-(Ce), which crystallized radially from carbonate or barite crystals, as well as the presence of halite and silicification within strongly brecciated units. However, bastnäsite-(Ce) inclusions in primary magmatic barite crystals have also been identified deeper in the Fe-carbonatite (below 1000 m), suggesting that primary crystallization of rare earth minerals occurred prior to hydrothermal leaching. Based on the intensity of hydrothermal brecciation, Cl depletion at depth and greater abundance of secondary fluid inclusions in carbonates in the upper levels, it is interpreted that hydrothermal activity was weaker in this deepest portion, thereby preserving the original magmatic textures. This early magmatic crystallization of rare earth minerals could be a significant factor in generating high-volume REE deposits. Crystallization of primary barite could be an important guide for REE exploration.

Orogenic gold deposits have provided most of gold to humanity. These deposits were formed by fluids carrying dissolved gold at temperatures of 200–500 °C and at crustal depths of 4–12 km. The model involves gold mobilization as HS⁻ complexes in aqueous solution buffered by CO₂, with gold precipitation following changes in pH, redox activity (fO₂), or H₂S activity. In this contribution, the involvement of carbonaceous organic matter is addressed by considering the formation of large and/or rich orogenic gold deposits in three stages: the source of gold, its solubilization, and its precipitation. First, gold accumulates in nodular pyrite within carbonaceous-rich sedimentary rocks formed by bacterial reduction of sulfates in seawater in black shales. Second, gold can be transported as hydrocarbon-metal complexes and colloidal gold nanoparticles for which the hydrocarbons can be generated from the thermal maturation of gold-bearing black shales or from abiotic origin. The capacity of hydrocarbons for solubilizing gold is greater than those of aqueous fluids. Third, gold can be precipitated efficiently with graphite derived from fluids containing hydrocarbons or by reducing organic-rich rocks. Black shales are thus a key component in the formation of large and rich orogenic gold deposits from the standpoints of source, transport, and precipitation. Unusual CO₂-rich, H₂O-poor fluids are documented for some of the largest and richest orogenic gold deposits, regardless of their age. These fluids are interpreted to result from chemical reactions involving hydrocarbon degradation, hence supporting the fundamental role of organic matter in forming exceptional orogenic gold deposits.

We provide the first large field investigation of more than 20 gold deposits and sites along the Keraf suture zone, covering an area of ~15,000 km². The area shows diverse structural settings for gold mineralization in greenschist, amphibolite, and hornfels metamorphosed rocks. The reverse fault and dyke-hosted settings appear to be the most favourable for forming gold deposits. Two major N-S-trending, ~300-km-long corridors of deformation are delineated (West and East) but only the West seems important for gold. From fluid inclusions, 6 types of fluids are distinguished based on specific volatile contents and proportions. Most types can be related to fluid evolution by hydrothermal reactions and phase separations from a primitive fluid of metamorphic origin, containing H₂O, CO₂, N₂, CH₄, C₂H₆, and H₂. Regional hydrothermal fields are identified based on the combinations of structural settings and fluid types. The northern field is defined as a single large hydrothermal system accounting for the mineralization of the 3 gold deposits (WG-03, UTM and Central). For sites along the West Corridor, such as Toubi, Anas, NW and Shereik, multiple and overlapping hydrothermal systems are necessary to explain the various fluid types and structural settings, even at the deposit scale (Shereik). Conversely, other sites (SW, Yasmine, Korup, Saijd, Dardora, Negeim, and SE) formed by localized individual hydrothermal systems. The documentation of fluids containing ethane at numerous sites and deposits is the most significant outcome. Such ethane-bearing fluids are generated by the metamorphism of carbonaceous-pyritic sedimentary rocks. These rocks are considered one of the best sources for providing ligands and gold for the formation of orogenic gold deposits. Consequently, the Keraf suture zone, composed mainly of carbonaceous turbidites, is interpreted to have provided such source rocks in the suture zone and below the adjacent cratons during the collision, forming orogenic gold deposits (graphical abstract).